The present invention relates to the remote inspection and servicing of heat exchangers, and particularly, the tubes in a nuclear steam generator.
Nuclear steam generators are huge heat exchangers in which tens of thousands of elongated tubes are secured at their open ends, to a thick, rigid tube sheet, at the lower head of the steam generator. Primary system water which has been heated in the nuclear reactor, is pumped into the lower head for passage through the tubes and return to the reactor. Secondary system water is supplied outside the tubes, for absorbing heat from the tubes to the extent that a phase change occurs, thereby producing steam for delivery to the turbine/generators.
Because the primary water passing through the tubes has a relatively high level of radioactivity as compared with the secondary water, and the secondary water is more likely to leak or be discharged from the plant, great care is taken to assure that primary water does not leak into the secondary water. One source of potential leakage is corrosion or other degradation of the steam generator tubes. Accordingly, during planned refuelling outages, a program of steam generator tube inspection and servicing is undertaken. Access to the tubes is through the tube openings at the tube sheet.
Over the years, the inspection and servicing techniques have evolved from the use of a team of human "jumpers", who would take turns to enter the steam generator head and perform the inspection and servicing for a brief period so as not to exceed radiation exposure limits, to the current use of robotic devices. These devices are temporarily attached to the tube sheet or adjacent structure, and carry an end effector which can perform inspection and/or servicing operations. The robot and end effector are controlled remotely, i.e., from outside the steam generator head.
Because the window of opportunity for steam generator tube inspection and servicing is limited not only by the plant outage, but also by the fact that other service may be required for the steam generator, it is becoming more and more important to reduce the time necessary for performing the inspection and servicing, and conversely to maximize the number of tubes which can be inspected and serviced during a particular outage. Ideally, the robot could be preprogrammed to inspect and/or service a predetermined number of specified tubes, on a continual schedule without interruption. This degree of automation is not possible, however, due to both practical and regulatory considerations. The robot is relatively bulky as compared with the size and spacing of the openings. With tens of thousands of openings, and a need for the inspection or servicing tool to register substantially exactly on the center line of the targeted opening, it is impossible with only a single calibration of the robot, to move the end effector over a large area of the tube sheet, and align the end effector with any one or more specified openings. As a consequence, the robot must be recalibrated frequently. Also, by observing the end effector in a camera, the operator frequently intervenes with a joy stick to toggle the end effector into registry with the targeted tube opening. Under regulatory procedures, independent confirmation must be achieved, that the end effector is in fact registered with the target opening. The inspection servicing step cannot be initiated, until such confirmation is obtained.
Given that the tube sheet presents tens of thousands of openings in a two dimensional pattern which is highly regular (except at the periphery and near structural features), both the main positioning technique and the position confirmation technique, require complex processing and are therefore sluggish.